2.8 How to Duplicate Collection without keeping it "linked"?
The feature to duplicate collections (without linking) has been available since commit e7ea99af1b0d933b9323be39d8b1c6f683bfba52. Therefore this feature was included in the release of 2.80 and subsequent versions. 

There are two options in the context menu of the Outliner for duplicating collections, Duplicate Collection and Duplicate Linked.



The screenshot shows the context menu of the Outliner in version 2.81

Duplicate Collection calls the bpy.ops.outliner.collection_duplicate() operator and does precisely what you intend. 


  Recursively duplicate the collection, all its children, objects and
  object data.




The old behavior with the linked object data is now called Duplicate Linked which calls the bpy.ops.outliner.collection_duplicate_linked().


  Recursively duplicate the collection, all its children and objects,
  with linked object data.